Neumann computers are not very suitable for solving multivariable optimization problems. However, to solve these problems, there is a method which uses an optimization apparatus (which could also be referred to as an ising device or a Boltzmann machine) that uses an ising energy function. The optimization apparatus performs calculation by replacing a problem to be calculated with an ising model, which is a model representing the behavior of magnetic spins.
The optimization apparatus determines the value (+1 or −1 (corresponding to the direction of a spin)) of an individual quantum bit, which will simply be referred to as a bit, included in an ising model, based on the values of the other bits, weight coefficients, each of which indicates a weight with respect to one of the other bits, and a bias. The weight coefficients and bias are constants determined based on the optimization problem to be calculated. For example, the optimization apparatus performs simulated annealing (SA) or the like to obtain, as a solution, a combination of bit values that achieves the minimum value of the above energy function.
See, for example, Japanese Laid-open Patent Publication No. 05-250346 and Japanese Laid-open Patent Publication No. 2017-138760.
In SA, in an individual trial that uses a different temperature, initial state, etc., update processing in which the value of one of a plurality of bits is updated is repeated many times. When an optimization apparatus that solves an optimization problem by using SA is realized by logic circuits, since a plurality of kinds of calculation processing are performed per update processing, a plurality of clock cycles are needed. Namely, the calculation efficiency of this kind of optimization apparatus is low.